Usually, the output power of VCSELs is strongly dependent on temperature and ageing condition. In order to avoid output power variations, some applications require sophisticated control or feedback mechanism like the usage of monitor photodiodes, for example.
Since the laser voltage depends on a lot of parameters and not only on the output power, the laser voltage usually is not a good indicator for output power.
It is known from Liu et al., Journal of Zhejiang University SCIENCE A, ISSN 1009-3095 that the laser voltage of VCSELs can empirically be described asV=(I*R0)/(T−T1)+Vt*ln(1+(I/(I0*(T−T1))))   (equation 1),
where I denotes the laser current, V the laser voltage and T the internal laser temperature i.e the temperature inside of the cavity. R0,T1,Vt and I0 are VCSEL characteristic parameters. In particular, these parameters are depending on production parameters such as the doping profile, active diameter, etc, and can be controlled only with low lot-to-lot precision.
The correlation between the internal laser temperature and the temperature
Ths of the heat sink is given byT=Ths+(V*I−Popt)*Rt   (equation 2).
In this equation, Popt denotes the laser output power and Rt the thermal resistance.
The term−Popt*Rt 
is known as photon-cooling effect, because the power of the emitted photons will not contribute to the heating of the laser. The power-dependency of the laser voltage V is then derived by combining equations 1 and 2.
Usually, the laser voltage cannot be used as a reliable indication for the output power of the VCSEL because the parameter R0, T1, Vt and I0 are generally not known.
For this reason, often monitor photodiodes are employed to monitor and stabilize the laser output power. However, the efficiency of monitor photodiodes is slightly temperature dependent and may deteriorate upon operation. Thus, the measured input parameter is dependent on the laser power, aging and ambient conditions and can distort the characteristics of the stabilization circuitry. To avoid this, stabilization circuitry often employs additional temperature measurement means thus complicating the design. A stabilization circuitry of this type is known for example from EP 1 085 624 A1.